1. Field of the Invention
The present invention relates to a semiconductor device including a capacitor provided above a semiconductor substrate and a plug which connects the capacitor and a diffusion area on a surface of the semiconductor substrate and a method for manufacturing the same.
2. Description of the Related Art
In recent years, much effort has been made to develop ferroelectric random access memories (FeRAM) that are nonvolatile memories utilizing a ferroelectric thin film. As a capacitor dielectric film in the FeRAM, a ferroelectric thin film is used which comprises a material such as PZT (Pb (Zrx, Ti1-xO3), BIT (Bi4Ti3O2), SBT (SrBi2T2O9), or the like.
For capacitor structures, much effort has been made to develop COP (Capacitor On Plug) structure having a capacitor placed on a plug (for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-289810). In the COP structure, a plug connected to an active area of a transistor is placed immediately below the capacitor. Thus, the employment of the COP structure enables a reduction in cell size as in the case of a stacked capacitor structure for a DRAM.
In a process of manufacturing an FeRAM, thermal treatment is carried out in an oxygen-containing atmosphere in order to recover from damage to the capacitor dielectric film. On this occasion, the plug under the capacitor is oxidized. The oxidized capacitor may increase the contact resistance between the plug and a lower electrode of the capacitor or cause the lower electrode to peel off.
To prevent an increase in contact resistance and peel-off of the lower electrode, attempts have been made to use a noble metal such as iridium, as a material for the lower electrode. This is because the noble metal does not allow oxygen to pass through easily and remains conductive even after oxidation.
On the other hand, silicon is often used as a material for the plug. In this case, since the noble metal is likely to react with silicon, a metal silicide film such as a titanium silicide film is provided between the plug (Si) and the lower electrode (noble metal) as a barrier film.
An oxide film is prone to be formed on a surface of the metal silicide film. Thus, an oxide film formed on the surface of the metal silicide film is removed by, for example, a process using a diluted fluoric acid before the lower electrode is formed.
In this case, the metal silicide film is etched at a lower etching rate than a surrounding silicon oxide film in which the plug is buried. As a result, as shown in FIG. 13, an upper part of a metal silicide film 81 on a plug 80 projects from a surface of a silicon oxide film 82. When there is such a projection of the metal silicide film 81 (a plug structure with a step) below an area in which the lower electrode is formed, the lower electrode is prone to peel off. Further, the projecting part of the metal silicide film 81 may be cracked. In this case, oxygen passes through the crack and reaches the inside of the plug 80. As a result, the plug 80 is oxidized to increase the contact resistance.